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Taito Spur
Mioc-Plioc acc. prism
Katsuura Basin
7 km
7 km
Quat acc. prism
Bando Deepsea Fan
Philippine Sea plate
Pacific plate
20 km
Fig. 4 Our reinterpretation of multichannel seismic profile C-C¢ of Iwabuchi et al. ( 1990 ). Ridges
and slopes between the Katsuura Basin and the Bando Deepsea Fan (Taito Spur) represent
Miocene-Pliocene and Quaternary accretionary prisms. Bold and fine dashed lines are reflectors
identified on the seismic profile
According to a submarine geologic map compiled by the Geological Survey of
Japan ( 1990 ), the area around the Boso triple junction has at least two strati-
graphic components: probable Miocene to Pliocene rocks (labeled Paleogene to
Miocene on the map), and Quaternary sediments. The latter are sediments scraped
from the Mogi Fan deposits in the area of the triple junction (Ogawa et al. 1989 )
(Figs. 3 and 4 ).
Thus, damming ridges (including the Taito Spur) have formed repeatedly and
are responsible for the sediment collapses that formed the two terraces. At each
stage, the formation of an accretionary prism was followed by the development
of a ponded basin to the west. When the dams collapsed, the resultant sediment
flows formed fan deposits in the area of the triple junction, similar to the present-
day Mogi Fan. The collapse of the dams was probably related to faulting at the
triple junction.
Transport of sediments from the Izu and Tanzawa areas along the Sagami trough
resulted in deposition of thick piles of clastic sediments at some places in the
middle of the trough floor (sediments such as those in the present-day Sagami
Basin and Middle Sagami Trough Basin), or on the eastern edge of the trough (like
those of the present-day Katsuura Basin), and one or more piles of sediment at the
triple junction (like those of the present-day Mogi Fan).
Seno et al. ( 1989 ) considered several models to explain the instability of the
triple-junction area on the basis of gravity data. They suggested that mud filling
the trench, or diapiric intrusion of serpentinite into the very base of the Izu-Bonin
trench, both of which are common to the south at the toe of the Izu-Bonin-
Mariana trench, may have caused the instability. They preferred a thick sedimen-
tary fill in the very deep basin as the source of the considerable negative gravity
anomaly. The deepest part of the recent trench-fill sediments attains a maximum
thickness of 4 km (based on line C-C¢ of Iwabuchi et al. 1990 ) (Fig. 4 ). Formation
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